The present invention generally relates to a method for setting an initial power supply voltage in a system having a programmable power supply voltage.
It has recently been regarded as desirable to dynamically adjust the power supply voltage and clock frequency of computer system processors to minimize power consumption and regulate heating of the processor core. The computer system processors themselves would seem to be an ideal mechanism for controlling these adjustments but for the fact that they must first receive the power and clock before they can determine the appropriate settings.
Until the processor is supplied with a minimum power-up voltage, it is not capable of driving the voltage identification outputs to control its operating voltage. Therefore it is necessary for the system hardware to ensure the processor is supplied with the required power-up voltage and to prevent the DC/DC converter from responding to the processor""s voltage identification outputs until the processor is driving them to select the startup voltage. To avoid damaging the processor, it is necessary to ensure that as the system is powered on, indeterminate signals from the processor do not cause the power supply voltage level to exceed the processors maximum operating limits.
The above issues are solved by a method and apparatus to provide a deterministic power-on voltage in a system having a processor-controlled voltage level. In one embodiment, the system includes a DC/DC power converter, a processor, and a selection circuit. The DC/DC converter receives a voltage setting signal from the selection circuit and provides an adjustable power output signal having a voltage indicated by the voltage setting signal. The processor is powered by the adjustable power output signal. When powered, the processor provides a programmable voltage setting signal. The selection circuit receives the programmable voltage setting signal, a hardwired voltage setting signal, and a selection signal, and when the selection signal is in a predetermined condition, the selection circuit provides the programmable voltage setting signal from the processor to the DC/DC converter. Preferably, when the selection signal is in a second predetermined condition complementary to the first predetermined condition, the circuit provides the hardwired voltage setting signal to the DC/DC converter. The first and second predetermined conditions of the selection signal are preferably de-assertion and assertion, respectively. The selection signal may be determined by a logic gate that combines a mode control signal and a power good signal, and causes the selection signal to select the voltage setting signal from the processor only when the power good signal is asserted and the mode control signal is de-asserted. This advantageously allows for the processor to dictate its operating voltage level, an ability that is extremely useful for power and thermal management in notebook PCs.